The advice below will drastically increase your level of security, but it will also mean greater effort in setup and administration
Highly recommended for paranoid personalities:

• Knowledge of social engineering and its principles
  Recognise that humans are the weakest link: this applies to the average user as well as professionals (ie: US Dept of Homeland Security)
• Access control at all levels, on all resources
• Use read only virtual machines to run untrusted code
• Increase verbosity of all processes, store logs backups in safe locations
• Use specialized software to monitor all activities and lower the threshold for warnings and alerts
• Know your enemy: constantly keep up to date with tools and techniques used by potential attackers
• Be aware of how complicated things can get (ie: port knocking backdoors?)

Too complicated? Use professional services, our job is to make it transparent.